1. Field of the Invention
The present invention relates to electrodeless discharge lamps and, more particularly, to circuit means for efficiently driving an electrodeless discharge lamp.
2. Description of the Prior Art
In U.S. Pat. No. 4,010,400, there is disclosed an electrodeless discharge lamp of the type including an ionizable medium within a sealed envelope including at least one particular ionizable gas at a given pressure capable of emitting radiant energy when subjected to a radio frequency field. An electric field having a magnitude sufficient to initiate ionization of the medium to form a radiation emitting discharge is coupled to the medium. Simultaneously, a radio frequency magnetic field for maintaining ionization is coupled to the medium. If the various parameters of the lamp are properly selected, a high efficiency electrodeless fluorescent lamp is theoretically possible.
It is known to drive the medium by means of an oscillator, which is usually crystal controlled, for generating an output signal at a given radio frequency, an RF amplifier responsive to the oscillator output signal, and an induction coil and a capacitor connected in series and responsive to the output of the amplifier. The coil is positioned in close physical proximity to the medium in the envelope for coupling to the medium the electric field and the magnetic field.
Such a lamp generally operates at 13.56 MHz, because the Federal Communications Commission permits such frequency to be used with great liberality. However, when operating at such frequency, a large number of problems are created. Thus, in spite of the promise of such an electrodeless discharge lamp, such promise has not been fulfilled heretofor because of difficulties in solving such problems.
The first problem is in selecting an amplifier circuit which will operate efficiently. That is, efficiently converting input energy into output power is essential if an electrodeless fluorescent lamp is to compete effectively with other types of lamps. A Class A amplifier has very low efficiency, generally less than 30%, rendering it unsuitable. A Class B amplifier has the potential of being 78.5% efficient, but, in reality, it generallly runs significantly less than this, rendering it unsuitable. A Class C amplifier is also very dependent on device-related parameters. In other words, a Class C circuit is quite sensitive to various capacitances within the circuit so that a Class C circuit does not lend itself well to mass production. Furthermore, transistors with ratings which are often three times whatever the DC input supply is are generally required and this presents significant problems.
A Class E amplifier has the potential of being 100% efficient because it functions as an on/off switch. Furthermore, it is not dependent on device-related parameters. On the other hand, it has an even worse voltage potential than a Class C amplifier in that a transistor rating of as much as four times the DC input supply is required, rendering the circuit impractical.
A Class D amplifier has the potential of being 100% efficient because it also functions as an on/off switch and the transistors require a rating of only 125% of whatever the DC input supply is. Furthermore, a Class D amplifier is typically not dependent on device-related parameters.
On the other hand, several factors suggest against the use of a Class D amplifier. First of all, Class D amplifiers have always required a load resistor and this results in a power loss. That is, it is generally considered that if a Class D amplifier drives a series resonant circuit, there is generally a short circuit at resonance, which would burn up the transistors. Thus, additional load circuits have always been added to prevent this from occurring.
Furthermore, if MOS devices are used for fast switching, there is always some inherent capacitance across the drain/source of the transistors and, considering that the output of each transistor is a square wave, this output capacitance has to be continuously charged and discharged. The generally accepted expression for calculating the losses inherent in charging and discharging this capacitance is 1/2CV.sup.2 f. However, for the purposes of this application, these losses will simply be referred to as CV.sup.2 f losses. Initially, available MOS devices had an output capacitance on the order of 400 pf. With a dc supply voltage of 140 volts and a frequency of 13.56 MHz, the CV.sup.2 f loss amounted to 53 watts dissipated power in charging and discharging the output capacitance. This is greater than the power one wants to consume for an entire light bulb. Even with more modern MOS transistors having an output capacitance of as low as 35 pf, one is theoretically still faced with an unacceptable 4.6 watts of loss. Therefore, it has been assumed that to use a Class D amplifier, a circuit would only work efficiently up to about 50 volts. Such a low voltage presents problems in initiating the discharge.
Still further, whenever a signal is generated at 13.56 MHz, there is the possibility of the generation of certain undesirable harmonics. If a switching type amplifier, such as a Class D or E amplifier, is used to drive an electrodeless discharge lamp, the output of the amplifier will be a square wave which is rich in harmonics. There is a harmonic at 54.24 MHz, a frequency close to that of television channel 2, a harmonic at 67.8 MHz, a frequency close to that of television channel 4, and harmonics at other odd multiples of the fundamental. Thus, there exists the possibility that the driving of an electrodeless discharge lamp at 13.56 MHz will create radio frequency interference (RFI) which will interfere with channels 2 and 4 of the TV band, as well as other allocated frequencies. This requires the addition of a filter network between the output of the amplifier and the series connected coil and capacitor.
Still further, if a discharge lamp of the above type is built without regard for the RFI problem, it will generally be found that the light level output is higher than desired. Thus, between the output of the amplifier and the drive induction coil, an impedance matching network that will set the level of the light output is necessary, such network matching the output impedance of the amplifier to the input impedance of the discharge. This requires an additional circuit. A satisfactory solution to all of the above problems has been unavailable heretofor.